Conductive metallization of substances without developing agents

ABSTRACT

A conductive metal layer is formed on a substrate having a softening point above about 200° C. by depositing copper and nickel particles having a substantially continuous oxide coating thereon on the substrate, and heating and pressing the metal particles at a temperature of at least 200° C. Unlike similar methods wherein oxide coated metal particles are used, no developing agent is required to render the metal layer conductive. The coated substrates are useful for a variety of uses such as EMI shielding and printed circuit boards.

RELATED APPLICATIONS

This is a continuation-in-part of Ser. No. 07/204,044, filed June 8,1988, now abandoned, which is a continuation-in-part of Ser. No.07/068,593, filed June 30, 1987, now abandoned.

BACKGROUND OF THE INVENTION

This invention related to the formation of a conductive layer of metalon a substrate. In particular, this invention relates to such a processwherein the conductive layer is formed from discrete metal particles ofcopper or nickel or combinations thereof. The invention alsoparticularly relates to such processes wherein the metal layer isrendered conductive without the aid of a chemical agent.

It is frequently desired to form a layer of conductive metal on a(generally) non-conductive substrate. Such composites are useful forprinted circuit boards, electromagnetic interference (EMI) shielding,and so forth.

Japanese publication JP110704(84) (Shin Gijutsu Kaihat) reports that aconductive layer may be formed on a substrate by spraying super fine(e.g.: 0.02 μm) metal particles with the aid of a carrier gas (e.g.:nitrogen or argon) onto a substrate, followed by low temperature (e.g.:80° C.) sintering. One significant disadvantage of this process is thelimited availability of such fine metal dust. Further, the use of metaldust in an air spray system requires extensive hygienic safeguards.

Japanese publication JP66133(73) (Fujimori Kogyo) and DE2,163,118(Fujimori Kogyo (Sakai)) both describe a process where metal particlesare mixed with a binder, applied to a substrate, dried, and treated withan agent which is an acid, a halogen, or a halogenide. Such processesare inconvenient and damaging to equipment in their requirement for abinder and a corrosive agent.

GB 2,085,340 teaches a paint containing copper particles and optionallya wetting agent (preferably triethanolamine), which is useful forproducing conductive coatings.

U.S. Pat. No. 4,434,084 (Dupont (Hicks)) and DE 2,411,988 (Dupont(Beske)) both describe processes wherein a mixture of copper and tinparticles are coated on a substrate with the aid of chemical activatorsand heat sufficient to melt the tin. In the U.S. patent the chemicalactivators are an organic acid flux and an organic amine in an inertorganic medium. In the German publication the chemical activator is aflux which may contain triethanolamine as an adjuvant.

Applicants' co-pending prior application, titled ConductiveMetallization of Substrates, serial number 068,593, filed June 30, 1987now abandoned claims a method of forming a conductive layer, whichmethod requires the use of a chemical developing agent.

SUMMARY OF THE INVENTION

The present invention pertains to a method of forming a conductive metallayer on a substrate, consisting essentially of:

(a) a depositing step, wherein a layer of metal particles comprisingcopper, nickel or a combination thereof, the particles having asubstantially continuous oxide coating on the surface thereof, isdeposited on a substrate having a softening point such that thesubstrate does not deform under processing conditions at 200° C.; and

(b) a heating and pressing step of subjecting the metal particles topressure at a temperature of at least 200° C. for a duration sufficientto improve the conductivity of the metal layer.

In another aspect, the invention is a coated substrate made by the abovemethod.

The method of the invention is convenient to carry out, involvesrelatively non-corrosive materials, and produces well adhered, veryconductive metal coatings on substrates.

DETAILED DESCRIPTION OF THE INVENTION

One element of the invention is the use of nickel particles, copperparticles, or combinations thereof. In general, anycommercially-available nickel or copper which is in the form of a powderis suitable for use in the invention. Such commercially-availablepowders typically have a substantially continuous oxide coating on thesurface thereof.

Since the oxide coating causes the metal-particles to be non-conductiveon the surface, it is necessary to remove the oxide or to disrupt theoxide surface, exposing conductive copper beneath, to achieve aconductive layer or pattern of conductive-particle toconductive-particle contact. While not critical, the use of aplate-shaped (lamellar) metal particle is preferred. However, excellentresults have been obtained with spherical particles. Of more importanceis the particle size of the nickel or copper particles. The principaldifficulty with large particle sizes is that of obtaining a uniformdistribution of particles on the substrate and ensuring adequateparticle-to-particle contact in a relatively thin metal layer.Generally, the particle size will be below 30, preferably below 20, andmore preferably below 10 μm.

The copper or nickel need not be particularly pure as previously stated,and typically oxides are present. Further, mixtures of copper or nickelwith other metals may be used. When mixtures are used, the mixture isdesirably at least 40, more desirably at least 50, preferably at least60, and most preferably at least 70 weight % copper and/or nickel.

The metal particles are deposited on a substrate. Suitable substratesinclude virtually any material which is capable of being in a relativelyhard, non-deformable state at processing conditions up to about 200° C.and of softening to a deformable state at higher temperatures so as toprovide for adherence of the metal powder to the substrate surface.Thus, materials such as those comprising borosilicate and otherlow-temperature softening glass and those comprising synthetic resinsare suitable, with those comprising synthetic resins being preferred.

Thermoplastic resins or polymers are typically employed as the syntheticresin of the present invention; metal powder is applied to the surfaceof a substrate comprising the resin, applying pressure to the thuslymetal-covered substrate while heating the substrate to a temperatureabove about 200° C., such that the metal layer becomes conductive andthe resin is so softened that the metal particles can be embedded intothe surface of the resin, and then cooling to ambient temperature.

Suitable thermoplastic resins include polyetherimides (PEI) (Ultem );and polyethersulfones (PES) (Victrex ) polyetheretherketone (PEEK),polyetherketone (PEK), polyarylate, polysulfone, polyarylsulfone, liquidcrystalinepolymers, and thermoplastic polyimide resins. It may be notedthat a common feature of such resins is a high degree of aromaticlinking units. The presence of such structure commonly produces polymerswhich have high glass transition temperatures and flow temperaturesabove 200.C. In general, it is desirable to have, in the repeating unitsof the polymer, at least 40%, desirably at least 50%, and more desirablyat least 60% of the carbon atoms being aromatic carbon atoms.

The substrates are illustratively in the form of thin (e.g.: 0.5 to 1mm) sheets but other forms (e.g.: cubes, spheres, etc.) would besuitable. The substrates may be selected from the usual commercialgrades of available materials, and no special handling or treatment isrequired. Although the surface to be coated should be relatively free ofdirt and grease, cleaning steps are not required unless the substratehas been subjected to an unusual amount of surface contamination.

It is specifically not a requirement of the invention that the metalparticles be contacted with a developing agent. However, the use of adeveloping agent can, in some instances, improve the conductivity oradhesion of the metal layer. Further information concerning theselection use of developing agents, as well as other relevantinformation, is contained in Applicants' aforementioned application,Ser. No. 07/068,593, now abandoned Conductive Metallization ofSubstrates, which is incorporated herein by reference. In theaforementioned application the developing agents are disclosed tocomprise:

(i) a long-chain aliphatic tertiary amine; (ii) a tertiary phosphine ora tertiary phosphite; or (iii) a bifunctional compound having both (1) afirst atom which is a trivalent nitrogen atom or a bivalent sulfur atomand (2) at least one second atom which is nitrogen or oxygen; the firstand second atoms being separated by at least two other atoms; thebifunctional compound being capable of forming a coordination complexwith copper or nickel, wherein such compounds include diamines,alkanolamines, secondary amino alcohols, tertiary amino alcohols,secondary sulfides, hydrazides, hydrazones, oximes and tertiary amides.

The metal particles are deposited on the substrate by any means whichwill achieve a layer of particles sufficient to provide a conductivesurface. A preferred method of deposition is to spray a dilutesuspension (about 5-15 weight % metal) of the metal in a volatileorganic solvent such as dichloromethane. For this purpose, ordinaryspray equipment such as used for painting may be employed.

Another preferred method is to coat the metal particles onto a transferplate (such as an aluminum plate, by spraying or other means, and thenpressing that plate onto the surface of the substrate. The transferplate is also preferably used as the molding plate (discussed furtherbelow). If the transfer plate has been milled or etched to form a raisedpattern, then the metal particles will form the mirror image of thatpattern on the substrate.

A mask can be used to form a pattern when metal is deposited by thedirect spray method or the transfer plate method. Other methods ofapplication such as roller coating, electrostatic coating, fluidized bedcoating, and so forth may also be used.

It is generally necessary to induce adhesion of the metal particles tothe substrate. To achieve desirable levels of adhesion of the metal, thecoated substrate is desirably subjected to heat and pressure.

One function of the heat and pressure is to cause the particles tophysically become embedded in the substrate. The heat required is thatwhich is sufficient to raise the temperature of the metalparticle-coated substrate to at least 200° C. for purposes of renderingthe metal particles conductive, and to a temperature sufficient to causeenough softening of the substrate that the metal particles can be forcedinto the surface of the substrate. Obviously, the temperature at whichthe substrate softens will vary considerably depending on the substratechosen. Molding temperature for a number of specific substrates arepresented in the Examples. The glass transition temperature (Tg) of asubstrate is a useful guide for choosing a molding temperature, butmolding temperatures are typically higher than the glass transitiontemperature of the substrate. If the molding temperature is too low,there will be insufficient flow of the substrate and lack of adequateadhesion. If the temperature is too high, the substrate will melt andperhaps degrade. Simple trial and error can easily determine the optimummolding temperature for any substrate.

Pressure is used in addition to heat to physically push the particlesinto the softened substrate. The precise amount of pressure is notcritical, and merely needs to be sufficient to deform the metalparticles to render them conductive. Typically molding pressures ofabout 5-100 MPa will be suitable.

The pressure may be provided by any conventional means. There is nominimum time for maintaining the pressure, except that the pressure mustbe maintained for a sufficient length of time to allow the substrate toreach at least 200° C., and optionally, the required moldingtemperature.

Whether or not heat is used to aid the adhesion of the metal to thesubstrate, heat is necessary to render the metal layer conductive. Thetemperature needed for rendering conductivity ("developing") can vary,but generally, a temperature of 200° to 400° C., preferably 220° to 350°C., and more preferably 240° to 280° C. is required

The exclusion of oxygen is believed to be important to the developmentof the metal to a conductive state; however, the use of a press to causethe metal to adhere to the substrate will inherently exclude oxygen andresult in no special means for excluding oxygen being required.

The process of the invention is subject to numerous other variations. Inparticular, the substrate may be a composite of several layers, withonly the top layer providing for adhesion of the metal. For instance, asheet of cured thermoset resin may be coated with a thin film ofthermoplastic resin, the metal added, and then the substrate heated andpressed.

The metallized substrates of the invention are useful in a wide varietyof applications including printed circuit boards, EMI shielding,capacitors, battery plates, electrical switches, and decorative panels.

The following examples are set forth to further explain the invention.Surface resistivity data presented in the examples is expressed in ohmsper square. A description of surface resistivity and techniques formeasurement can be found in ASTM D 257-78 (Reapproved 1983).

EXAMPLE 1

The following data illustrates the principle that commercially-availablecopper powders have an oxide coating on the surface thereof whichrenders the copper powder nonconductive.

The copper powder used in the examples which follow was grade 22BB400obtained from POUDMET in France. This is a commercial flake form with98% by weight being smaller than 40 microns, and having a specific areaof 3,400 cm^(2/) g.

(1) Untreated Poudmet 22BB400 copper flake was slurried in CH2ClCH2Cland coated onto PEI film. The surface resistance of this coating layerwas greater than 10⁶ ohms/square.

(2) Untreated Poudmet 22BB400 copper flake was subjected to treatmentwith hydrogen in an autoclave at 220°-250° C/20-40 psi (as described inU.S. Pat. No. 4,614,837 to Kane et al., issued Sept. 30, 1986, to removeoxide layers which may coat the flake/particle) until no furtherhydrogen uptake was observed.

(3) The treated copper flake from (2) was slurry coated onto PEI film,as described in (1) and had a surface resistance of 0.6 ohms/square.

This shows that hydrogen treatment to remove oxide layers which coat thesurface of the flake or particle renders the particle conductive. Thus,the non-conductivity of the particles prior to treatment is attributedto the presence of such oxide layers.

(4) The slurry coated samples of (3) were placed in an air oven at 71°C. and the surface resistance was assessed at intervals.

    ______________________________________                                         0 hours           0.6 ohms/square                                             24 hours          1.7 ohms/square                                             72 hours          3.0 ohms/square                                            192 hours         20.0 ohms/square                                            ______________________________________                                    

Thus, the layer of flakes was reoxidizing with time, with anaccompanying increase in resistance. Of course, free flakes which arecontacted with oxygen (in a manner not controlled by diffusion rate tothe same extent as the coated layer described above) would be expectedto oxidize much more rapidly, and a layer of flakes subsequentlyprocessed to embed the layer in a thermoplastic would be expected tooxidize much more slowly if at all.

According the the present invention, it is possible to use oxide-coatedmetal particles of copper and/or nickel without the need for a step suchas this hydrogen treatment to remove the oxide coating from the metalparticles.

EXAMPLE 2 Comparative

This data illustrates that for substrates not within the scope of theinvention (those which soften below 200° C.), good (high) conductivity,can be achieved only in the presence of a suitable developing agent.

Part A: A performed plaque of high-impact polystyrene (HIPS) about3"×4"×0.03" thick (76×102×0.76 mm) was sprayed with a suspension ofcopper powder in dichloromethane and dried in an oven at 55° C. (toevaporate the solvent completely). The copper applied weighed about 0.3g and exhibited slight adhesion to the plastic as a result of solventswelling of the surface during spray up. The coated preform could thusbe manipulated without the coating falling off, but if the coating wasrubbed the copper became detached. At this stage surface resistance ofthe coating was greater than 10⁷ ohms/square. The coated preform wasthen compression molded between chromed steel plates at 215° C. for 12min under 5 tons/square inch (69 MPa) pressure. The molds were cooled ina second press at 20° C. and the plastic removed. The copper powder hadbecome compressed into the surface of the plastic (since the molding wasdone at above the temperature at which the polymer became fluid) to forma well adhered continuous copper layer. This layer was orange in colorand had a surface resistance of greater than 10⁶ ohms/square.

Part B: a HIPS preform was sprayed with copper powder as in Part A,dried, and then sprayed with a 5% solution ofN,N-bis(2-hydroxyethyl)cocoamine (a developing agent) in dichloromethaneso as to afford a 3:1 weight ratio of Cu:amine after drying in an ovenat 55° C. to evaporate the solvent. The coated preform was compressionmolded as in Example 2A. After development the well-adhered coppercoating was pink in color and possessed a surface resistance of 0.01ohms/square. The use of the developing agent thus improved theconductivity by a factor of 10⁸.

EXAMPLE 3

A. Preforms were prepared from polyethersulfone (Victrex 4100P, ICI) bymolding at 240° C. Attempts to incorporate N,N bis(2-hydroxyethyl)tallowamine (a developing agent) into the PES by extrusion resulted innon-homogenous, somewhat brittle, samples.

B. A series of PES preforms 4"×4"×0.03" were sprayed with a suspensionof copper powder in dichloromethane to give a weight gain of about 0.25g of copper after solvent evaporation. The preforms were compressionmolded at various temperatures between 150° C. and 250° C. for 10 min at5 tpsi (69 MPa) pressure. After cooling, the copper layers were examinedfor conductivity by the four-probe method, and for adhesion by means oftape attached to the metal surface. Following this the samples were cutin half and one half sprayed with a solution of N,N-bis(2-hydroxyethyl)tallowamine in dichloromethane to give Cu:amine ratios of around 2:1 to4:1 and then molded a further 10 min. After cooling the conductivity andadhesion were reassessed. Below 220° C. adhesion of the copper layer wasgenerally poor due to non-flow of the PES, and the copper could easilybe scratched away. At 200°-220° C. there was some evidence whichindicated that treatment with the amine improved the adhesion of thecopper. Above 220° C. adhesion of the copper coatings was much improved,particularly in the presence of developer, due to good flow of the PES.Use of the amine in samples molded at below 220° C. led to copper layersexhibiting enhanced conductivity (below 0.1 ohms/square) but ofinsufficient adhesion. Of particular interest however, was theobservation that at temperatures above 200° C., copper powder sprayedonto PES polymer produced copper layers of high conductivity, having asurface resistance of 0.03 to 0.08 ohms/square, in the absence of adeveloping agent.

C. PES pellets were subjected to solvent extraction in a Soxhletapparatus for seven days using a mixture of xylene and chloroform(90:10), and then pressed into preforms. These were sprayed with copperpowder, and in some cases also with N,N-bis(2-hydroxyethyl)tallowaminedeveloper and compression molded at 220° C. No difference inconductivity of the copper layer with or without the amine was observed(0.01 ohm/square) but use of developer enhanced the adhesion of thecopper to the PES. These results indicate that the reason for theability of PES to afford highly conductive layers of copper, whensprayed with copper powder and compression molded, is probably not dueto the presence of polymerization catalyst residues or of phosphoruscontaining antioxidants.

D. PEI pellets were solvent extracted as in (C) and molded intopreforms. The PEI preforms were sprayed with a suspension of copperpowder and compression molded 10 min. at 5 tpsi (69 MPa) pressure atvarious temperatures. It was observed that solvent extraction did notprevent PEI from affording conductive copper layers, having a surfaceresistance of 0.095 to 0.11 ohms/square, in the absence of a developerwhen sprayed with copper powder and molded at 220° C. It may be notedthat use of an N,N-bis(2-hydroxyethyl) tallowamine developer at 220° C.and above improved the adhesion of the copper layer.

E. PEI preforms were sprayed with a suspension of nickel powder (Alcan756) in tetrahydrofuran and dried After being compression molded for 10min at 230° C. under 5 tpsi (69 MPa) pressure, and cooled the preformwas found to bear a grey layer of nickel which exhibited highconductivity (a surface resistance of 0.15 ohms/square) in many areas.When similar experiments were carried out at 180° C. adhesion of thenickel to PEI was inadequate. These results indicate that the attainmentof highly conductive nickel coatings upon PEI do not necessarily requirethe use of a developer.

EXAMPLE 4

The following examples illustrate the principle of transfer molding asapplied to the process of the invention.

A. A 1/4" (6.3 mm) thick aluminum block was milled to produce a positiveimage of the circuit pattern required for a simple circuit based on theNE555 timer integrated circuit upon the raised portions. This patternpiece was sprayed with a suspension of copper powder in dichloromethaneand then compression molded in contact with a 0.03" (0.76 mm) thickpiece of polyetherimide for 10 min at 220° C. under 5 tpsi (69 MPa). Byfilling the hollow areas of the aluminum pattern piece with a Vitonrubber filler of the appropriate thickness, the PEI remained relativelyflat after the transfer molding.

Upon cooling it was found that the copper powder had transferred fromthe aluminum to the surface of the PEI to form an orange circuitpattern. Corner to corner resistance over 5" (127 mm) or so of 3/16"(4.8 mm) wide track was around 100-200 ohms using 2 ohmmeter probes.Better adhesion of the metal (due to improved PEI fluidity) was obtainedby molding at 240° C. but this produced some "flashing" (i.e.: oozing ofthe resin out of the mold).

B. A 0.03" (0.76 mm) thick piece of polyetherimide which bore a circuitpattern formed by transfer molding of copper from an aluminum pattern inthe absence of developer at 200° C. and possessing a resistance of 6ohms over a 3" (76 mm) length of

1/8" (3.2 mm) wide copper track using 2-probes was placed in anevacuated tube containing 6 drops of N,N-bis(2-hydroxyethyl) cocoamineand the tube placed in an oven for 30 min at 200° C. After cooling theresistance of the same track measured over th same distance had beenreduced to 0.1 ohm/square and the color of the copper changed fromorange to pink. This indicates the process of the present invention canbe combined with a subsequent step wherein the circuit pattern istreated with a developer to improve conductivity. This two-stepprocedure avoids the need for incorporation of the developer into thesubstrate material, avoiding substrate mechanical property performancereduction of the type observed in Example 3A.

Only a limited number of preferred embodiments of the invention havebeen described above. However, one skilled in the art will recognize thenumerous substitutions, modifications and alterations which can be madewithout departing from the spirit and scope of the invention as limitedby the following claims.

We claim:
 1. A method of forming a conductive metal layer on asubstrate, consisting essentially of:(a) a depositing step, wherein alayer of metal particles comprising copper, nickel or a combinationthereof, said particles having a substantially continuous oxide coatingon the surface thereof, is deposited on a substrate having a softeningpoint such that said substrate does not deform under processingconditions at 200° C. and (b) a heating and pressing step of subjectingmetal particles to pressure at a temperature of at least 200° C. for aduration sufficient to improve the conductivity of said metal layer. 2.The method of claim 1 wherein said depositing step comprises sprayingsaid metal particles onto said substrate.
 3. The method of claim 2wherein said spraying is done with the aid of a liquid suspension agent.4. The method of claim 3 wherein said liquid suspension agent is anorganic solvent.
 5. The method of claim 1 wherein said copper, nickel orcombinations thereof contains less than about 40 weight % ofnon-conductive layer forming metals.
 6. The method of claim 1 whereinsaid copper, nickel or combinations thereof contains less than about 20weight % of non-conductive layer forming metals.
 7. The method of claim1 wherein said metal particles have a number average particle size ofless than about 30 μm.
 8. The method of claim 1 wherein said metals arepresent in the form of a mixture containing less than about 25 weight ofother non-conductive layer forming metals.
 9. The method of claim 1wherein said metal particles are in the form of flakes.
 10. The methodof claim 1 wherein said heating and pressing step takes place at 200° C.to about 400° C.
 11. The method of claim 10 wherein said heating andpressing step takes place at about 220° to about 350° C.
 12. The methodof claim 1 wherein said heating and pressing step takes place attemperature below the melting point of any substantially present metal.13. The method of claim 12 wherein the pressure is 5-100 MPa.
 14. Themethod of claim 1 wherein the maximum temperature in said heating andpressing step is at or above the softening point of said substrate. 15.The method of claim 1 wherein said substrate is comprised of a syntheticresin.
 16. The method of claim 15 wherein said synthetic resin is athermoplastic resin wherein, in the repeating units of the polymer, atleast 40% of the carbon atoms are aromatic carbon atoms.
 17. The methodof claim 16 wherein said resin is a polyether sulfone, a polyetherimide,a polyetheretherketone (PEEK), a polyetherketone (PEK), polyarylate, apolysulfone, a polyarylsulfone, a liquid crystaline polymer, or athermoplastic polyimide resin.
 18. The method of claim 1 wherein saiddepositing step comprises applying said metal particles to a transfersurface and pressing the transfer surface against said substrate. 19.The method of claim 18 wherein said transfer surface has relief areaswhich will not contact the substrate, thereby transferring a pattern ofmetal particles
 20. The method of claim 1 wherein a metal layer has beendeposited on two opposite surfaces of the substrate.
 21. A coatedsubstrate produced by the method of claim
 1. 22. A coated substrateproduced by the method of claim
 20. 23. A method of shielding a spacefrom a source of EMI, comprising placing the coated substrate of claim21 between the source of EMI and the space to be shielded.
 24. A methodof shielding a space from a source of EMI comprising placing the coatedsubstrate of claim 22 between the source of EMI and the space to beshielded.